In wireless communications systems, wireless terminals and base stations are designed to transmit and receive radio frequency (RF) signals that propagate in RF environments. Depending on the type of wireless communications system and the services offered by the wireless system, the wireless terminals and base stations are equipped to perform specific signal-processing functions. For example, some wireless systems are required to identify the geographical location (i.e. geolocation) of the wireless terminals communicating on the system. Such wireless systems have been referred to as geolocation systems. The term geolocation as used herein refers to the point in two- or three-dimensional space defined by a set of coordinates, e.g. longitude and latitude, and/or defined by a vector, i.e. distance and direction, from a known point in space.
Some conventional geolocation systems identify the geolocation of a wireless terminal by determining, at a plurality of locations, the time-of-arrival of the line-of-sight component of a signal transmitted by the wireless terminal. They then process the various times-of-arrival to determine the distance of the wireless terminal from each of, for example, three receiver locations, and from this "distance" information, the geolocation system determines the geolocation of the wireless terminal itself
The line-of-sight component of the transmitted signal is that component of the signal that propagated directly from the wireless terminal to the location at which the signal was received (e.g. a base station) without scattering or reflecting off structures in the RF environment. The term scattering refers to the phenomenon wherein an RF signal, traveling in an RF environment, hits and reflects of structures in the RF environment, thereby causing the RF signal to take random paths through the RF environment. This so-called multipath phenomenon can cause the incoming signal to be composed of several repeated versions of the transmitted signal, each version being a multipath component of the incoming signal.
To determine the time-of-arrival of the line-of-sight component of the transmitted signal, such geolocation systems typically receive the transmitted signal and pass the so-called incoming signal through a matched filter. The matched filter generates a correlation value based on a comparison of the shape of the waveform of the incoming signal to the shape of the waveform of the transmitted signal. The correlation value essentially peaks each time the matched filter determines that the shape of the waveform of the incoming signal is similar to or matches the shape of the waveform of the transmitted signal. Each time the correlation value reaches a peak value, the geolocation system identifies that time as the time-of-arrival of a multipath component of the incoming signal. Since the line-of-sight component of the transmitted signal travels directly to the location of the receiving unit, such conventional geolocation systems assume that the time-of-arrival of the line-of-sight component of the incoming signal is the time at which the correlation value reaches its first peak.
These geolocation systems, however, are hindered by their failure to consider the effects of scattering or the scattering hostility, i.e. the propensity of the RF environment to scatter an RF signal traveling in the RF environment, on the incoming signal. Depending on the scattering hostility of the RF environment in which the incoming signal traveled, the line-of-sight path and thus the line-of-sight component of the incoming signal may never reach the receiver location. For example, there may be a large building in the RF environment that blocks the line-of-sight path between the transmitting unit and the receiving unit, thus preventing the line-of-sight component from ever reaching the receiving unit. When this happens, the time-of-arrival of the first-identified peak of the correlation value may not be the time-of-arrival of the line-of-sight component of the incoming signal, but rather the time-of-arrival of a later-arriving multipath version of the transmitted signal.
When this happens, the identified time-of-arrival of the line-of-sight component will be so-called "time-shifted." That is, identified time-of-arrival will be the time-of-arrival of the later-arriving multipath component. As a result, the conventional geolocation system will incorrectly assume that the time-of-arrival of the line-of-sight component of the incoming signal arrived at a later time than it would have arrived if it had not been blocked as described above. Thus, the conventional geolocation system would incorrectly calculate the geolocation of the wireless terminal based on a "time-shifted" time-of-arrival.